The present invention relates to a process for injection molding information recording disks wherein the predrying of a material resin is dispensed with. It also relates to a process for injection molding thin information recording disks wherein the plasticating of a material resin and the injection and filling thereof into a mold are separately accomplished.
Injection molding of information recording disks, such as optical disks, magnetic disks, magneto-optical disks, or the like has conventionally utilized an in-line screw type injection molding apparatus, which is equipped with an injection screw rotatable and movable back and forth in a heating cylinder having a nozzle at the front end and a feed port at the rear.
Traditionally, less expensive, unpelletized resins can hardly be used as feedstock for the injection molding. The unpellitized resins might thus be pelletized into pellets of a predetermined size for use as feedstock for the injection molding. Forming the pellets from resins is called pelletization and is carried out by means of an extruder with a vent. The extruder has a heating cylinder to melt the resin. The molten resin is extruded into strand and the strand is then cut into a predetermined length.
Since a resin to be used for such injection molding is of small-grained, granulated sugar type and contains unreacted monomers, there arises a problem in molding such a resin. The resin is therefore melted by heating and is processed into pellets of a certain grain size before being used as a material resin. Since this resin as prepared into the pellets is hygroscopic, it must be predried to remove moisture before being used for molding. Conventionally, the molding material has a water content of from 0.005% to 0.01% by the steps of pelletizing process and preliminary drying process. These predrying steps must be carried out in the prior art, or the physical properties of the molded article are degraded due to hydrolysis. However, the moisture which may remain in the material resin can not only be the cause of molding failures such as silver streaking or yellowing, but as well result in the hydrolysis of the resin when being plasticated to cause a decrease in the strength of the molded disks. Also, an ill-managed dryer may allow fine foreign matters in the air to be absorbed, and such matters can be, for example, carbonized and surface as black spots on the molded articles, thereby rendering the articles defective. When the molded articles are information recording disks, such as optical disks, magnetic disks, magneto-optical disks, or the like in particular, extremely small and few black spots would cause disorders in their reading.
In addition, in a typical prior art molding process, the material resin, as such, will be subjected to at least three times of heating including pelletization, predrying, and injection molding. This extended heat history can easily cause a deterioration in physical properties, especially in strength. Transparency may also be impaired.
Further, when a conventional in-line screw type injection apparatus is utilized, a material resin fed into a cylinder through a feed port located at the rear of the cylinder is melted and kneaded, that is, plasticated, by the revolutions of an injection screw, the screw being moved backward by the pressure of the resin. As such, the resin will be metered within the front portion of the cylinder, and the metered material will be injected and filled by the advancement of the screw into a cavity of a mold to which a nozzle tip abuts to produce the desired information recording disks. For the purpose of improving transferability and birefringence in thin disks (approximately 1.2 mm in thickness) as well, compression molding which compresses a resin injected and filled into a mold is sometimes practiced.
In such molding in which the plasticating of a resin and the injection and filling thereof are carried out by a single screw, however, since the metering is accomplished by moving an injection screw backward by means of the pressure of the plasticated resin, dispersion in the metering will tend to show as uneven areas despite the use of a small injection apparatus for molding disks or the like which involves small amounts of resin. It will result in an inconstant amount of injection and filling into a cavity, possibly producing accidental no-filling or overfilling and will prevent constant molding for a long period of time.
In addition, unevenness of temperatures will occur throughout the plasticated resin. Moreover, an L/D ratio will change due to the backward movement of the injection screw, giving a tendency that the resin temperature at the completion of metering is lower than that at the beginning. Solutions to these problems include the adoption of a screw with mixing means or a spiral barrier type screw, etc.; however, the effects of such solutions have so far been insufficient and improvements are desired.